1. Field of the Invention
The present invention relates to an aluminum alloy having a fine crystal structure and thereby having excellent mechanical properties, particularly strength and elongation.
2. Description of the Related Art
Al-Mg-Si aluminum-based alloys, particularly 6000-series aluminum alloys such as 6061, 6066, 6070 and 6082 are conventionally hot- or cold-worked or plastically formed by forging, rolling, extruding, etc. The 6061 alloy is most popular in such use, but has a relatively low tensile strength of 27-33 kgf/mm.sup.2 and is used as a medium strength material.
The 6000-series alloys are plastically formed to a desired form having increased strength and then subjected to a heat treatment such as T.sub.6 treatment comprising a solution heat treatment and a subsequent artificial ageing under a condition providing the highest aged strength. The heat treatment, however, coarsens the recrystallized crystal grains generated during the hot plastic working and thereby reduces the mechanical properties, particularly strength and elongation. The coarsening of recrystallized grains is particularly evident when worked at a high reduction or working ratio of 50% or more.
Japanese Unexamined Patent Publication (Kokai) No. 1-283337 proposed suppressing the grain coarsening by using the additive elements of Mn, Cr, Zr, etc., in which it is stated that Mn, Cr and Zr when jointly added to Al-Mg-Si aluminum-based alloys in a certain amount, suppresses the grain growth otherwise occurring during forging or other forming processes and during heat treatments and thereby provides a plastically formed article having a refined crystal structure.
It is a current trend that materials applied to automobile parts such as the frame and suspension members require a tensile strength of 40 kgf/mm.sup.2 or higher and an elongation of 15% or more when plastically formed and T.sub.6 -heat-treated. The above-proposed aluminum alloy, however, does not satisfy this requirement because of poor mechanical properties involving tensile strength, proof strength and elongation, although it has improved characteristics resulting from the refined crystal structure in comparison with other existing materials.
The conventional practical aluminum alloys, such as the 6000-series alloys, do not provide a tensile strength of 40 kgf/mm.sup.2 or higher and an elongation of 15% or more when a cast material is hot- or cold-worked and T.sub.6 -heat-treated, the hot- and cold-working being usually effected by forging or rolling with or without an antecedent hot-extrusion.